The present invention generally relates to liquid crystal display devices and more particularly to an in-plane switching liquid crystal display device. An in-plane switching liquid crystal display device is a device driven by an electric field acting parallel to the liquid crystal layer forming the liquid crystal display device.
Conventionally, driving of a liquid crystal display device has been achieved by applying an electric field to a liquid crystal layer confined by a pair of substrates such that the electric field acts perpendicularly to the liquid crystal layer. On the other hand, there is a proposal of a so-called in-plane switching (IPS) liquid crystal display device, in which an electric field is applied to the liquid crystal layer such that the electric field acts in the direction parallel to the substrates. In such an IPS liquid crystal display device, an interdigital electrode is provided on one of the foregoing substrates.
FIGS. 1A and 1B show the principle of such an IPS liquid crystal display device.
Referring to FIG. 1A, a liquid crystal layer 13 containing therein liquid crystal molecules is confined between a pair of mutually opposing glass substrates 11 and 12 in such a manner that the liquid crystal layer makes an intimate contact with a molecular alignment film 11A covering the substrate 11 and also an intimate contact with a molecular alignment film 12A covering the substrate 12. Further, polarizers 11B and 12B are disposed at respective outer sides of the glass substrates 11A and 11B in a crossed Nicol state. Further, a pair of electrodes 14A and 14B are provided on the glass substrate 11 in a state that the electrodes 14A and 14B are covered by the molecular alignment film 11A.
In the non-activated state of FIG. 1A, there is no driving voltage applied across the electrodes 14A and 14B and the liquid crystal molecules 13A of the liquid crystal layer 13 are aligned in a predetermined direction in a plane generally parallel to the substrates 11 and 12.
In the activated state of FIG. 1B, on the other hand, a driving voltage is applied across the electrodes 14A and 14B, and an electric field is induced in the liquid crystal layer 13 in the direction generally parallel to the liquid crystal layer 13. As a result of the electric field, the direction of the liquid crystal molecules 13A, or molecular orientation, is changed. An IPS liquid crystal display device achieves the desired optical switching by using such a change of the molecular orientation of the liquid crystal molecules 13A. Due to the fact that the change of the molecular orientation occurs in the plane parallel to the liquid crystal layer 13, an IPS liquid crystal display device generally provides a superior viewing angle as compared with the conventional twist-nematic (TN) liquid crystal display devices.
On the other hand, such an IPS liquid crystal display device, lacking an electrode on the opposing substrate 12 contrary to a conventional TN liquid crystal display device, tends to induce polarization in the molecular alignment film 12A, while such a polarization induced in the molecular alignment film 12A tends to cause the problem of image sticking or afterimage, in which the represented image tends to remain after the image has been changed. This problem of image sticking becomes particularly acute when the liquid crystal display device is used to display an image for a prolonged time period.
In order to eliminate the problem of image sticking, it is necessary to use a low-resistance liquid crystal having a resistance lower than the resistance of the liquid crystal used in a conventional TN liquid crystal display device, for the liquid crystal layer 13. However, such a liquid crystal having a low resistance generally has a large dielectric constant and tends to dissolve impurities. In other words, a low-resistance liquid crystal is vulnerable to contamination. Such a contamination may come from the sealing material of the liquid crystal display device or from the molecular alignment film. Once the liquid crystal is contaminated, the representation performance of the liquid crystal display device is severely deteriorated.
Further, it should be noted that the electric field 13B induced in the liquid crystal layer 13 in the driving state of the liquid crystal display device is not exactly parallel to the plane of the liquid crystal layer 13 in the vicinity of the electrode 14A or 14B. This means that the electric field component parallel to the plane of the liquid crystal layer 13 becomes small and the response speed of the liquid crystal molecules 13A becomes accordingly small in the vicinity of the electrodes 14A and 14B.
Thus, there is an acute demand of improved performance for the conventional IPS liquid display device.